This invention relates generally to removal of gaseous sulfur dioxide from flue gas by scrubbing with a sodium based alkaline liquor which absorbs sulfur dioxide then regenerating or recausticizing the resulting spent scrubber liquor with a calcium compound such as slaked lime or limestone. Such systems are often referred to as dual alkali systems because the scrubber liquor utilizes sodium as the alkali in scrubbing while calcium is utilized to regenerate the sodium compounds.
For purposes of discussion, the essential equations involved in a system using lime and limestone are:
In the scrubber:
(1) Na.sub.2 SO.sub.3 SO.sub.2 +H.sub.2 O--2NaHSO.sub.3 (Some Na.sub.2 SO.sub.4 is also produced) PA0 (2) 2NaHSO.sub.3 +CaCO.sub.3 --CaSO.sub.3 +Na.sub.2 SO.sub.3 +CO.sub.2 +H.sub.2 O PA0 (3) H.sub.2 O+Na.sub.2 SO.sub.4 +CaCO.sub.3 --CaSO.sub.4 +2NaOH+CO.sub.2 PA0 (4) 2NaHSO.sub.3 +Ca(OH).sub.2 --CaSO.sub.3 +2H.sub.2 O+Na.sub.2 SO.sub.3 PA0 (5) Na.sub.2 SO.sub.3 +Ca(OH).sub.2 --CaSO.sub.3 +2NaOH PA0 (6) Na.sub.2 SO.sub.4 +Ca(OH).sub.2 --CaSO.sub.4 +2NaOH
and
In the recausticizing stage:
Make-up sodium in the form of soda ash may be added to the liquor after recausticizing is completed or, in accordance with this invention, may be added with the lime and limestone.
A typical scrubber bleed liquor to be recausticized has a pH in the range of pH 5.5 to pH 6.0.
The present invention provides an improved scrubbing system that greatly improves scrubber efficiency; and it also provides an improved recausticizing process enabling achievement of the improved scrubber operation while still using relatively inexpensive limestone as a major source of calcium.
Traditionally, slaked lime has been employed in recausticizing because of its reactivity, but in the last decade improved systems have been developed in which relatively inexpensive limestone serves as the calcium source. A process of this type is disclosed in U.S. Pat. No. 4,462,969 in which process the recausticizing is carried out in separate, serially connected sludge blankets under conditions whereby ground limestone is retained sufficiently long to effect substantially complete utilization thereof, while only a relatively short liquor residence is required. In such earlier limestone process, the pH of the regenerated (recausticized) liquor is maintained well below pH 7.0, preferably about pH 6.2. This, because above about pH 6.5 the reaction rate of limestone drops and an unacceptably increased residence time of liquor and sludge is required to utilize the limestone and recausticize the liquor. Should a modestly higher pH be desired in a limestone system it may be achieved by greatly exceeding the stoichiometric limestone quantity, but this is impractical because of higher costs.
In contrast, when lime is used as the calcium source the end pH of the system and resulting regenerated scrubber liquor is in the broad pH range of pH 8.0-pH 13.0 but almost invariably in a range of about pH 11.0-pH 13.0. Thus, depending on whether the system is limestone or lime, prior scrubbers operate with inlet liquor that is either well below pH 7.0 or well above pH 8.0.
Both of the foregoing lime and limestone systems are in commercial use and are the subjects of continuing research and development to increase efficiency with resultant cost savings.